OBJECTIVES: The aim of this retrospective investigation wasto devise a protocol for obtaining consistent myocardial counts for patients attending myocardial perfusion imaging. METHODS: A total of 229 patients were included in the study sample. For each rest single-photon emission computed tomography data set, a three-dimensional ellipse was manually defined to isolate the left ventricle. The count rate per voxel was measured from a threshold region of interest covering the normally perfused myocardium (70-100% ofmaximum counts). Data were normalized for net administered activity and plotted against patient weight. The correlation between the variables was used to derive aprotocol for adjusting injected activity or scan time to obtain consistent counts from patient to patient. RESULTS: There was a significant correlation between counts per second per voxel per MBq and patient weight. The best fit was achieved with an inverse model. A correlation equation was derived for patients weighing more than 60 kg (combined R=0.79): [CPS/voxel/MBq=(0.0163/weight(kg)]-3.68×10+A in which A, a sex-dependent constant, was 0 for men and -2.920×10 for women. CONCLUSION: There was a significant inverse correlation between patient weight and CPS per voxel per MBq.